1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
A projection exposure apparatus is used in a lithography process in a semiconductor device manufacturing process. The lithography process is the process of projecting and transferring a circuit pattern of a semiconductor device onto a substrate (called a silicon substrate, glass substrate, wafer, or the like) to be formed into a semiconductor device. Recently, with advances in the miniaturization of semiconductor devices, patterns with line widths of 0.15 μm or less have been transferred. Decreasing the sizes of semiconductor devices can increase the integration degrees of semiconductor devices and manufacture low-power, high-performance devices. The demand for further decreases in the sizes of semiconductor devices is high. This also increases the demand for increases in the resolving power of projection exposure apparatuses.
According to the Rayleigh expression, a resolving power RP, a wavelength λ of exposure light, and the NA of a projection exposure apparatus satisfy the following relational expression. In the following description, NA represents the numerical aperture of a projection optical system unless otherwise specified.RP=k1·λ/NA where k1 is a proportionality constant.
As indicated by the Rayleigh expression, as a method of increasing the resolving power of a projection exposure apparatus, a method of increasing the NA of a projection optical system and decreasing the wavelength of exposure light is available. There has recently been an increasing tendency toward an increase in the NA of a projection optical system and a decrease in the wavelength of exposure light. In the tendency toward shorter wavelength of exposure light, for example, a KrF excimer laser (wavelength: 248 nm) has been used for exposure, and an ArF excimer laser (wavelength: 193 nm) has also been used as a light source for critical layer exposure.
With an increase in NA, polarized light needs to be used for an illumination optical system. The detailed arrangement of a polarized illumination is disclosed in the pamphlet of International Publication WO 2004/051717, Japanese Patent Laid-Open No. 05-109601, and the like.
The glass materials used for projection optical systems using lasers having wavelengths in a high-energy exposure light wavelength range of 300 nm or less have been limited to SiO2 (to be referred to as quartz hereinafter) and CaF2 (to be referred to as fluorite hereinafter).
In order to increase the NA, an immersion exposure technique is used. This technique achieves an NA of 1.0 or more by using quartz as a glass material for a projection lens and filling the space between the bottom lens group of a projection system and a wafer with a liquid having a higher refractive index than air. In this specification, the “bottom lens group” means lenses or a cemented lens, of the lenses of a projection optical system, which is located nearest to a wafer and in contact with an immersion liquid. The “cemented lens” means a lens obtained by cementing a plurality of lenses. Pure water having a refractive index of 1.44 is to be currently used as an immersion liquid. Using pure water as an immersion liquid can increase the NA of the projection system up to an NA of 1.44 theoretically and up to about 1.3 constructionally.
In order to further increase the NA, there have been proposed a technique of using a liquid having a higher refractive index than pure water as an immersion liquid and a technique of using a glass material having a higher refractive index than quartz for the bottom lens group of a projection optical system. It is expected to increase the structural NA to about 1.4 by using a liquid having a higher refractive index than pure water as an immersion liquid and increase the NA to about 1.65 by using a glass material having a higher refractive index than quartz for the bottom lens group of the projection optical system. A medium having a refractive index of about 1.8 has been thought as a liquid having a higher refractive index than pure water. However, there has been no explicit description about the details of such a medium, for example, its composition and physical properties other than the refractive index.
U.S. Pre-Grant Publication No. 2006/0198029 discloses an example which limits an illumination condition to tangential polarized light and uses a uniaxial crystal such as Al2O3 as a glass material having a higher refractive index than quartz. However, for example, the practical use of uniaxial crystals cannot be expected, and the illumination condition is limited. This makes this technique impractical.
As a glass material having a higher refractive index than quartz, Lu3Al5O12 (to be referred to LuAG hereinafter) having a refractive index of 2.14 is currently in the spotlight.
Quartz which is generally used as a glass material for a projection optical system using an ArF excimer laser has no structural directivity. For this reason, no intrinsic birefringence originating from the crystal structure is observed, and only birefringences observed are a stress birefringence at the time of processing and a birefringence due to a residual stress caused by an annealing temperature distribution in a manufacturing process or the like. The magnitudes of these birefringences are about 1 nm/cm or less per lens in terms of pv value, and tend to decrease owing to manufacturing and processing techniques. In contrast, fluorite or LuAG as a crystal glass material has a birefringence originating from its crystal structure. The magnitudes of the birefringences of fluorite and LuAG are respectively 3.4 nm/cm and about 30 nm/cm at maximum.
FIG. 1 shows a birefringence distribution obtained when the <1, 1, 1> crystal axis of a crystal structure indicated by the isotropic crystal material of a flat plate is oriented in a direction perpendicular to the drawing surface, and a birefringence distribution obtained when the <1, 0, 0> crystal axis is oriented in a direction perpendicular to the drawing surface. Referring to FIG. 1, the angle at which a light beam passes through is the radial direction, and the position at which the light beam passes is expressed by an angle of deviation. Referring to FIG. 1, the length of each short line represents a relative birefringence amount, and the direction represents the direction of an axis along which the birefringence moves.
Referring to FIG. 1, the birefringence is 0 in the <1, 1, 1> crystal axis direction and the <1, 0, 0> crystal axis direction, and exhibits the maximum value in the <1, 1, 0> crystal axis direction. It is a challenge for an exposure apparatus to solve the problem of birefringences originating from this crystal structure. As described above, these crystal glass materials are often used for the bottom lens group of a projection lens because of their structures. In addition, in general, the R2 surface (on the wafer surface side) of a bottom lens group is flat or has a large curvature to allow easy control of a liquid.
FIG. 2 plainly shows part of a projection optical system. Reference numeral 100 denotes a state in which light passing through the bottom lens group of the projection optical system and an immersion liquid forms an image on a wafer surface; 110, a bottom lens group; 120, an immersion liquid; and 130, a wafer surface. Reference symbol OA denotes an optic axis; O, a point at which the optic axis OA intersects the wafer surface 130; and P, a point on the wafer surface which is spaced apart from the point O by Xi. Light beams 10, 11, and 12 reach the point O. Reference numeral 11 denotes the principal ray; and 10 and 12, the light beams reaching the point O on the wafer surface at an incident angle θ. Light beams 20, 21, and 22 reach the point P. Reference numeral 21 denotes the principal ray; and 20 and 22, the light beams reaching the point P on the wafer surface at the incident angle θ.
The incident angle θ is expressed by the following relational expression by using an NA and a refractive index Ni of an immersion liquid.NA=Ni sin θ
When, therefore, the <1, 1, 1> and <1, 0, 0> crystal axes in FIG. 1 are oriented to the optic axis, as the NA increases, the birefringence originating from a crystal structure has a larger influence.
When a glass material having a refractive index of 2.0 or more, for example, LuAG, is used for the bottom lens group of a projection optical system, it is necessary to correct a birefringence (to be referred to as an intrinsic birefringence or IBR) originating from the crystallinity of the glass material. Techniques of correcting birefringences are disclosed in Japanese Patent Laid-Open Nos. 2006-113533 and 2004-45692.
The technique disclosed in Japanese Patent Laid-Open No. 2006-113533 corrects birefringences by using a physical property that when calcium oxide or magnesium oxide is used for a bottom lens group and its adjacent lens group, birefringences originating from crystal structures exhibit opposite signs between the glass materials. However, since the transmittance of such a material is low, and the IBR of calcium oxide is equal to or more than 300 nm/cm, the correction residue increases. For this reason, this technique is not practical.
Japanese Patent Laid-Open Nos. 2006-113533 and 2004-45692 disclose methods of orienting the <1, 0, 0> and <1, 1, 1> crystal axes to the optic axis of the projection optical system.